Although the SI unit for energy is the joule energy transferred to homes is in the order of several hundreds of thousands of joules so the kilowatt-hour is used as a measure of energy instead.As an example, a kettle has a power rating of about 2-3kW.
1kWh is the energy transferred by a device with a power rating of 1kW operating for 1 hour.1kWh = 3.6MJ
Energy transferred = power × time.The kWh is also called a unit by electricity companies. The cost per kWh depends on the electricity company.To work out how much energy a device has used you multiply the power in kW with the time it is used for in hours.To work out the cost you multiply the energy in kWh by the cost per unit.